Processes for forming shaped articles from particulate mixtures are known in the art. Classically, a desired particulate material is mixed with a binder and then formed into the desired shape, this being called a green body. The green body is then fired to provide a fusion of the particulate material and to drive off the binder, thereby producing the desired shaped product with proper surface texture, strength, etc. Modern methods include press and sinter (P&S) and powder injection molding (PIM). In P&S, a mixture of one or more of a metal, metal oxide, intermetallic or ceramic powder and a small amount of binder (about 5% of the powder volume) are placed in a relatively simple mold, pressed into a green body, and then sintered. The small amount of binder is decomposed during the sintering step, so a separate step of removing the binder is not necessary. However, P&S is limited to simple parts.
In PIM, a mixture of one or more of a metal, metal oxide, intermetallic or ceramic powder and a quantity of binder from 30% to 60% of the volume of powder are heated to a liquid state and then injected under pressure into a mold to form a part. Once in the mold, the binder is removed in one or more separate steps and the part is fired to sinter the particles into a solid part. PIM is capable of producing quite complex parts.
In the production of shaped objects by PIM in the manner above described, it has been found that the binder, while necessary to the process, creates problems. The binder must be used in order to form an object of practical use, but most of it must be removed before the part can be sintered, although in some cases a portion of the binder remains until sintering is completed.
Direct removal of the PIM binder during sintering is problematic. Many binders leave behind ash upon decomposition. When such ash combines with certain ingredients in the powder component, eutectic mixtures may be formed. Such eutectic compounds as TiC may be formed from titanium and carbon ash, and these can result in serious problems in the formed part.
Thermoplastic binders which decompose on heating have been used. However, these materials tend to soften or melt first and then decompose, creating problems on decomposition. Thermoplastic materials have been tried which decompose below their melting point and thereby remain in place until decomposition. Binders have been removed by exposure to a decomposing atmosphere, such as an acid atmosphere to decompose an acid-labile organic binder. The drawback of this approach is the use of an acid atmosphere, requiring a special chamber and hazardous material handling capabilities. Similar binders which are subject to catalytic decomposition have been used, such as a polyacetal. The drawback of this approach is that the decomposition product is formaldehyde, which also requires special equipment to collect and decompose the formaldehyde.
The prior art has recognized this problem and has therefore attempted to remove the binder from the shaped green body prior to the step of firing. Such processes have used various solvents, including organic solvents, triple-point CO2, and water to dissolve and remove the binder. While systems using such procedures can provide advantages over procedures wherein the binder is removed during firing, articles formed by removing the binder prior to firing still have the tendency to crack during the binder removal as well as during the firing operation. One reason for this is that the binder is removed from the green body by means of a solvent when the binder is in the solid state, and upon dissolution the binder, the binder-solvent mixture has a tendency to expand. This problem has been approached by various means, including heating the green body prior to exposing it to the solvent, by using a solvent to remove a portion of the binder and removing the remainder by firing, and by using a two-part binder, each part of which is soluble in a different solvent, so each solvent removes a portion of the binder, and by using the different solvents in a stepwise manner. Each of these methods includes its own drawbacks.
Thus, the need remains for binders which are useful, particularly in powder injection molding, which require a minimum number of steps to remove, which have high thixotropic energy, which melt at a low temperatures, which provide a green body having high strength, and which decompose thermally to yield simple, environmentally safe products, substantially free of ash, thereby yielding a binder which performs its function but which provides a process of powder injection molding which proceeds with a minimum number of process steps, can be carried out in an air atmosphere in many cases, and does not leave behind deleterious residues, either in the part or in the environment. The present invention requires only simple, standard equipment which is inexpensive and commonly available. The steps of debinding and sintering may be carried out in the same equipment, on a continuous basis, thereby avoiding downtime for cooling and transfer from debinding equipment to sintering equipment.